Filters for separating frequencies are widespread within technology. Electrical and optical filters are used, in particular, within the field of telecommunications. The general purpose of these filters is to isolate and/or transform a wide spectrum of frequencies present at their input and to make this available at their output.
An important criterion for the quality of a filter is its efficacy in the exclusive transmission of the desired frequency range. A good filter will block, as precisely as possible, those frequencies that lie outside the desired range and allows the frequencies within the desired range to pass through practically unimpeded, i.e. with no loss.
The loss factor and selectivity, which can be given simply by the ‘Q-factor’ (also known as the quality factor) of the filter, are thus important characteristics of filters. Normally, high-quality filters having low losses are desired.
These criteria are particularly relevant, especially within the field of telecommunications technology where messages are transmitted at high frequencies, low power and over large distances.
A frequently-used physical principle for producing filters is based on resonance; the corresponding filters are accordingly called resonance filters. For example, they are supplied as waveguides or in a coaxial design. Although filters of this type satisfy the technical requirements of high quality and low loss, they are large, heavy and expensive. Furthermore, it is difficult to combine filters of this type with the planar design of conventional circuitry.
‘Cavity resonance filters’ or planar cavity filters are used, in particular, to overcome the drawback of size and tall overall height. For example, the cavities in resonance filters of this type are produced by etching from silicon wafers. Two different designs are known in this instance. In accordance with a first design the cavities are produced by wet etching a plurality of individual wafers. The worked wafers are then placed on top of one another, positioned precisely relative to one another and permanently interconnected. The different layers are coupled via openings in a metal layer covering the wafers. In accordance with a second design the cavities are produced by RIE (reactive ion etching). This method also makes it possible to produce the coupling openings, required for coupling individual wafers, together with the cavities and in the same wafer.
A drawback of the first design is that it is necessary to adjust a large number of (for example five or more) wafers that must be arranged and positioned precisely above one another. A drawback of the second design lies in the high costs associated with manufacture by RIE, since the advantage of cost-effective manufacture by wet etching techniques cannot by utilized, despite the use of silicon as a base material.